Holoprosencephaly is a complex abnormality of the forebrain resulted from failure in the diverticulation of the prosencephalon. It is classified in three major varieties:

Alobar: single primitive ventricle; fused thalami; absence of the third ventricle, neurohypophysis, olfatory bulbs and the head is generally small;

Semilobar: the two cerebral hemispheres are partially separated;

Lobar: the interhemispheric fissure is well developed posteriosly and anteriorly, but there is still a variable degree of fusion of the cyngulate gyrus and the lateral ventricles and absence of the septum pellucidum.

Chromossomal abnormality has been associated, specially trisomy 13, but maternal infections and paternal toxic exposures also can be implicated. All kinds of holoprosencephaly can be diagnosed by ultrasound. In some cases, it can potentially identify affected fetuses in the first trimester. Nevertheless, when the sonographic diagnosis is uncertain in cases shuch as maternal obesity, fetal size and position, the MR imaging can be helpful.

Case report

We demonstrate some ultrasound, MRI and postnatal images related to a case of alobar holoprosencephaly.

A 34-year-old woman (gravida 1, para 0), was referred at 18 weeks of gestation with a ultrasound diagnosis of alobar holoprosencephaly. The ultrasound demonstrated a small head, hypotelorism, primitive ventricle, fused thalami, absent falx and septum pellucidum, cleft lip and palate. All of them confirmed posteriorly by MRI at 20 weeks. Karyotype was offered but refused by the patient. The pregnancy miscarried at 23 weeks. Pathological examination of the fetus confirmed alobar holoprosencephaly.